# How do we know what atomic number an element has???

Dec 6, 2015

If you know what the element is called... look it up.

You can always get the atomic number, as long as you know what element you're looking at. Always. You can see the atomic number directly on each square. The relevant information from each square is:

$\text{Z}$
$\text{X}$
$\text{Name}$
$\text{Atomic Mass}$

$\text{Z}$ is the atomic number.
$\text{X}$ is the atomic symbol.

For example, Phosphorus ($\text{P}$) is denoted as element number $15$. So of course, the atomic number must be $15$. Its atomic mass is about $\text{30.97 g/mol}$, which is close to $31$, so the most common isotope has a mass number of $31$.

If the element is generally known to be stable, the most common isotope is the most stable.

So, we have:

$\text{number of protons + number of neutrons = mass number}$

$\text{atomic number = number of protons}$

$\text{mass number - atomic number = number of neutrons}$

$31 - 15 = \text{16 neutrons}$

Finally, the number of electrons has to be equal to the number of protons for an element in its elemental form, i.e. its natural, neutral state. So it must have $15$ electrons as $\text{P}$.